The antilock brake system (ABS) for use in efficient and safety braking without locking of wheels of automobiles was included in 45% of automobiles produced in Japan in 1996, and in more than 90% in 2009. At present, the antilock brake system is included in most of automobiles.
The widespread antilock brake system is intended, for example, to detect the rotation speeds of wheels by a rotation speed detector (wheel speed sensor), calculate the acceleration and the deceleration and estimate the vehicle speed and the slip ratio by a controller, and drive an actuator to control the brake fluid pressure based on the calculation and estimation results.
Bearing device with such a rotation speed detector in a roll bearing for supporting automobile wheels (hub bearing) is also widely used. There is a bearing device structured such that a seal member is fitted to the inner side of opposed surfaces of a rotational inner ring and a fixed outer ring, a magnetic encoder with N and S poles alternately arranged at regular intervals in the circumferential direction is attached to the inner ring at the inner side of the seal member, and a magnetic sensor is attached to the outer ring so as to be opposed to the magnetic encoder to detect the rotation of the magnetic encoder (for example, refer to Patent Document 1).
In the structure of the bearing device as described in Patent Document 1, muddy water or magnetized iron pieces may enter between the magnetic encoder and the magnetic sensor, which causes damage to the magnetic encoder and the magnetic sensor or changes in magnetic property of the magnetic encoder. In addition, the rotation torque of the bearing device increases due to sliding resistance of the seal member positioned at the outer side of the magnetic encoder.
To solve these problems, there is a bearing device in which a cup-shaped protective cover is press-fitted into the inner end portion of the outer ring to cover the magnetic encoder from the inner side without the seal member (for example, refer to Patent Document 2).